In conventional reflection-type liquid crystal display devices, display modes such as TN (Twisted Nematic) mode and STN (Super Twisted Nematic) mode are used. However, when such a display mode is used, it is necessary to provide polarizers over the surfaces of the liquid crystal panel, and absorption of light by the polarizers disadvantageously decreases the light utilization efficiency.
A newly-proposed display mode which does not require the use of polarizers is a mode in which display is performed by switching the liquid crystal layer by application of a voltage across the liquid crystal layer between a light-scattering state in which light is scattered by the liquid crystal layer and a light-transmitting state in which light is transmitted through the liquid crystal layer (hereinafter, referred to as “scattering-transmitting mode”). In this display mode, for example, a Polymer Dispersed Liquid Crystal (PDLC) is used.
A liquid crystal layer which uses a PDLC (PDLC layer) includes a plurality of liquid crystal regions (or “liquid crystal droplets”) dispersed in a polymer material. The liquid crystal regions are formed in spaces defined by polymer walls (hereinafter, referred to as “small sections”). In the PDLC having such a structure, when no voltage is applied (i.e., in the absence of an applied voltage), there is a difference in refractive index between the liquid crystal in the liquid crystal regions and the polymer, so that light is scattered by the interfaces between the liquid crystal regions and the polymer, resulting in a white display state. When a voltage is applied across the PDLC layer (i.e., in the presence of an applied voltage), the alignment of the liquid crystal changes so that the liquid crystal and the polymer have generally equal refractive indexes, allowing light to be transmitted through the PDLC layer. If, in this situation, there is a light-absorbing plate on the rear side of the liquid crystal layer, the transmitted light is absorbed by the light-absorbing plate, resulting in a black display state.
In place of the light-absorbing plate, a reflective layer which is configured to selectively reflect light of a specific color may be provided on the rear side of the liquid crystal layer. For example, when the PDLC layer is a forward-scattering type liquid crystal layer, display can be performed with a specific color and black (mirror reflection) by providing, for example, a smooth metal plate (mirror) which shows the specific color. When the PDLC layer is a backward-scattering type liquid crystal layer, display can be performed with a specific color and white by providing, for example, a metal plate which shows the specific color.
Alternatively, a retroreflective layer may be provided on the rear side of the liquid crystal layer. For example, Patent Document 1 discloses a display device which is based on a combination of a retroreflective layer and a PDLC (retroreflection-type liquid crystal display device). Hereinafter, the structure of a conventional display device which uses a PDLC is described with an example of the retroreflection-type liquid crystal display device.
FIG. 11 is a schematic cross-sectional view of an active matrix driven retroreflection-type liquid crystal display device disclosed in Patent Document 1. The display device 300 includes a front substrate 110 that includes a color filter 119, a transparent counter electrode 111, and an alignment film 112, a rear substrate 109 that is arranged to oppose the front substrate 110, and a liquid crystal layer (PDLC layer) 113 that is interposed between these substrates 110, 109. The rear substrate 109 includes a plurality of switching elements (TFT) 101, an insulating layer 102 overlying the switching elements 101, which has a surface structure that exhibits a retroreflection property, a plurality of reflective electrodes 105, and an alignment film 118. The reflective electrodes 105 are provided on the insulating layer 102 and have elevations and recesses which are in conformity with the surface shape of the insulating layer 102. The plurality of reflective electrodes 105 are arranged in respective ones of the pixels, each of which is a unit of displaying of images, such that they are mutually separated. Each of the reflective electrodes 105 is coupled to the drain electrode of a corresponding one of the switching elements 101 via a contact hole formed in the insulating layer 102. The alignment film 118 is provided over the insulating layer 102 and the reflective electrodes 105 and has elevations and recesses which are in conformity with the surface shape of the insulating layer 102.
Next, an operation of the display device 300 is described.
In the display device 300, the liquid crystal layer 113 is switchable between a light-transmitting state in which light is transmitted through the liquid crystal layer and a light-scattering state in which light is scattered by the liquid crystal layer (forward scattering and backward scattering) by varying a voltage applied between the counter electrode 111 and the reflective electrodes 105. When the liquid crystal layer 113 is controlled to be in a light-transmitting state, light incoming from a light source outside the display device or from an environment is transmitted through the front substrate 110 and the liquid crystal layer 113 and then reflected by the reflective electrodes 105 to travel back to the side it came from. Here, an image that reaches a viewer from the display device is the eyes of the viewer, so that a “black” display state is obtained.
When the liquid crystal layer 113 is controlled to be in a light-scattering state, light incoming from a light source or environment impinges on the liquid crystal layer 113 at the front substrate 110 side and is then scattered by the liquid crystal layer 113. When the liquid crystal layer 113 is a forward-scattering type liquid crystal layer, the scattered light is reflected by the reflective electrodes 105 and, then, the reflected light travels through the liquid crystal layer 113, which is in a light-scattering state, and outgoes toward the viewer. Due to scattering of light by the liquid crystal layer 113, the retroreflection property of the reflective electrodes 105 is canceled so that incoming light cannot travel back to the side it came from. Thus, a “white” display state is obtained.
Note that, although the display device 300 shown in FIG. 11 includes the reflective electrodes 105 which function as both the retroreflective layer and the pixel electrodes, the retroreflective layer may be a separate component from the pixel electrodes. The retroreflective layer may be interposed between the pixel electrodes and the rear substrate 109 or may be provided on the rear side of the rear substrate 109.
A reflection-type liquid crystal display device which uses a scattering-transmitting mode, such as the display device 300, disadvantageously has a lower display contrast than display devices which use other display modes.
To increase the brightness of the white display state of the display device 300, it is necessary to increase the thickness of the liquid crystal layer 113 so that light impinging on the liquid crystal layer 113 can be scattered more assuredly. However, when the thickness of the liquid crystal layer 113 is increased, a higher voltage needs to be applied across the liquid crystal layer 113 for switching the liquid crystal layer 113 to a light-transmitting state. If the voltage applied across the liquid crystal layer 113 is insufficient, the transmittance of the liquid crystal layer 113 decreases, so that a high quality black display state cannot be obtained.
Patent Documents 2 to 4 disclose a technique of improving the display contrast by controlling liquid crystal directors in a guest-host mode liquid crystal display device which uses a PDLC that contains a dichromatic dye in a liquid crystal. In these patent documents, the liquid crystal regions are deformed to expand along a plane of the substrate to have an oblate shape, so that the directors of liquid crystal molecules in the absence of an applied voltage across the liquid crystal layer can be controlled to be oriented parallel with the substrate. For example, Patent Documents 2 and 3 disclose a technique of mechanically pressing the liquid crystal layer such that the liquid crystal droplets have an oblate shape which is elongated along a direction parallel with the substrate. Patent Document 4 discloses a technique of imposing a shear stress in parallel with the substrate in formation of the liquid crystal layer and a technique of curing only superficial part of the liquid crystal layer.
With the techniques of Patent Documents 2 to 4, the display contrast can be improved without increasing the thickness of the liquid crystal layer. More specifically, in a conventional liquid crystal layer which includes a PDLC, the liquid crystal directors in the liquid crystal regions are oriented in random directions in the absence of an applied voltage. When a voltage is applied, these liquid crystal directors are oriented perpendicular to the substrate. On the other hand, in the techniques of Patent Documents 2 to 4, the liquid crystal directors can be controlled to be oriented parallel to the substrate in the absence of an applied voltage, and therefore, the difference between light absorption achieved in the absence of an applied voltage and light absorption achieved in the presence of an applied voltage (when the liquid crystal directors are perpendicular to the substrate) can be increased. As a result, the display contrast can be improved as compared with the conventional example.